With the interest in 43 foot untuned verticals, and some of the less than accurate claims being made, I thought I’d share the results of a 25 foot untuned vertical antenna I developed a few years ago. Hopefully, I can shed some more light on its practical use, as well as show how to gain a bit more performance.
The 25 foot vertical is qualitatively similar to the 43 foot vertical, but at half size, and is designed to provide acceptably efficient operation from 40 meters to 10 meters. So, the information here will roughly translate over to 80 through 20 meter operation of the 43 foot vertical (or, even better as you will see, a 50 foot vertical).
I started with the same general idea that the 43 foot vertical starts with – maximum frequency of operation limited to an antenna length of about 5/8 wavelength, above which lobing occurs and low angle radiation suffers.
In addition to ground losses, efficiency is largely determined by feedline loss, and in general, high VSWR starts killing efficiency at the upper end where VSWR starts hitting 20 plus, and at the low end where radiation resistance falls and where VSWR really skyrockets. There are excellent write-ups about this by WX7G here and VK1OD here (I’m new to this board, so my apology to others I may have missed!)
So, my goal was to design a system with less than 3 dB overall loss (excluding tuner loss) and has close to 0 dBi performance by:
• Reducing VSWR at the high end to reduce feedline loss in the 20 plus VSWR region.
• Reducing VSWR at the low end to reduce feedline loss in the 50 plus VSWR region.
• Using a minimalistic ground system.
• Retaining lobe-free low angle radiation over the entire frequency range.
(Note – EZNEC model segmentation was standardized for consistency with “minimum recommenced” at 40 MHz for all data presented here. It’s not hard to get a dB or two different answer if differing segmentation is used.)
Lowering the VSWR Throughout the Frequency Range
Not surprisingly, the first 2 goals are met by lengthening the antenna radiator – VSWR at the low end drops and radiation resistance increases as we start approaching ¼ wave, while VSWR also drops at the high end in the region of ¾ wave. As a consequence, feedline losses drop. Also, as the antenna is lengthened, we relax the requirements placed on the ground system.
Here is the effect of lengthening the antenna from 21.5 feet to 25 feet:
Re-Lowering Radiation Angle at High Frequencies
But of course, lengthening the antenna beyond 5/8 wavelength means we are killing low angle radiation at the high end of the frequency range as shown here for 29 MHz:
What to do…. What to do… In words, we want the antenna to have the desirable low angle radiation of the short 21.5 foot antenna, while retaining the desirable VSWR characteristics of the 25 foot antenna. We need a frequency dependant shortener!
The “shortener” is just a grounded 4 foot mid loaded spike near the base of the radiator which is tuned to a bit above the highest frequency of operation (10 Meters). The idea is to have lots of current flow at 10 Meters (and to a lesser extent at 12 Meters) which effectively shortens the antenna and brings down angle of maximum radiation, while have little current flow, and little effect, at low frequencies.
Here is the radiation pattern at 29 MHz with the “shortener”, showing decent low angle radiation. Gain at 20 degrees elevation goes up to 3.6 dB (compared to 0.6 dB without the “shortener”):
And, at the low end we see that the input impedance remains mostly unaffected (and, with God and Mr, Maxwell smiling upon us, the high end VSWR actually becomes more favorable), which shows that we still have the desirable impedance and VSWR characteristics of the full 25 feet:
A Miminalistic Ground
A big topic, with lots of possibilities. Much has been written about ground – some of it is even true ☺.
Here is the approach I took for this design – A raised feed point (5 feet in this case), and a long metal fence as ground/counterpoise. Raising the feed point relaxes the requirement on the ground system by moving the counterpoise away from lossy earth. There are a few nice write-ups regarding raised feedpoint verticals.
Most or all of them assume use of a set of tuned radials. A really nice and efficient system, but it is starting to become a pretty complex and real-estate consuming project to hoist a radial system up in the air.
Always being one with a eye toward Madman Muntzing, I did the modeling and analysis of a long metal fence for use as a grounding system.
It’s always a matter of concern and debate as to how one measures and compares performance of one system over another. The 2 metrics I usually use are:
• How do the R losses of the system over “real” ground compare to the same system over “perfect” ground.
• How does the real-earth far-field absolute gain of the system compare to a similar reference system at the same height?
Fence Ground with “real” earth (0.01S/M) vs. Fence ground with “perfect” ground – this gives us an idea of how much loss is contributed to the system by ground resistance:
Feedpoint resistance increases only a moderate amount at 40 Meters, and results in 2.2 dB of real-world loss due to Fence ground system resistance. Good enough for my purposes given that the alternatives are a much more complex ground/counterpoise system.
Fence ground with “real” earth vs. 50 radial with “real” earth, at same feedpoint height – this gives us an idea of how the system performs compared to a similar, but more complex, “optimum” system:
The far-field gain difference between the Fence ground system (-0.8 dBi) and the 50 radial system (1.8 dBi) is only 2.6 dB – a value which I find to be perfectly acceptable in trade-off with ground system complexity.
How Good is my EZNEC model?
EZNEC and other modeling software is great, but do they actually reflect reality?
The qualified answer is – Yes!, provided one tweeks the model to reflect reality. The model I used for this project was tweeked by using a small element at the base of the antenna to force EZNEC to actually feed the antenna near the base (oddly enough, this is quite critical to get model results that match measurements), and by shifting around “real” ground conductivity, in my case to 0.01 S/M.
Here is the side-by-side comparison of my EZNEC model vs. Measured data (HP8753 vector network analyzer, 64 averaging) for the 25 ft vertical:
No system, especially one the operates with high VSWR over much of it’s range like the 25 ft vertical (or the 43 foot vertical) is complete without considering feedline loss. Again, there are very good write-ups out there by WX7G and VK1OD and others.
I my case, I had a nice chunk of 7/8” heliax laying around, and just used that for the 15 meter long feedline.
Here is the feedline loss (using VK1OD’s enhanced calculator) and worst case assumptions:
It’s nice to have heliax laying around ☺. If you are thinking of buying some line for a system like this, you may want to consider the old reliable LMR-400, but in it’s 75 ohm version. Using 75 ohm line buys us a about 1 dB in reduced line loss. Of course, you could put a remotely tuned Tuner at the antenna, but that can pretty pricey pretty quick.
The Whole System
Shown below is the system performance of the whole system (sans Tuner, which is a whole-nuther topic in and of itself):
Including feedline loss (but excluding the tuner), the overall gain is:
Note that I have included 5 MHz data. We might be able to get away with using the system on 5 MHz, but 3.8 MHz is not really practical. We could add a loading coil to for 80 Meters (160 Meters for the 43 foot antenna) to lower the feedline loss, but coil Q would be an issue, and the shear magnitude of the voltage extremes are pretty problematic (ask me how I know ☺ ).
A very simple system is shown here that:
• Is long enough to provide low VSWR at the low and high ends of the frequency range
• Has a “shortener” that prevents high angle lobing and radiation at 10, and to a lesser extent at 12 Meters, while enhancing efficiency at the 40 Meter low end.
• Has low feedline losses over entire 40 to 10 Meter range.
• Has reasonably near 0 dBi gain over entire 40 to 10 Meter range.
• Uses an existing metal fence line as a fairly efficient ground/counterpoise.
• Decent low angle radiation over entire 40 to 10 Meter range.
It can’t get much simpler:
Dave Benzel – KD6RF – 2016 Oct 16
3,345 total views, 2 views today